Many control devices rely on analog setpoints to indicate a desired state to which a system should be controlled. An analog setpoint is typically a voltage or current applied to a controller that represents a desired value of a measured parameter. The voltage/current may represent a desired value of a temperature, motor speed, pressure, pressure differential, temperature differential or other parameter. The analog setpoint is typically digitized at the controller and converted to a setpoint value for the parameter. The setpoint value can be compared to measured values of the parameter for control purposes. For example, a temperature controller can receive an analog signal of 2.2 Volts, digitize the signal and convert the value to 20 degrees Celsius. The controller can then compare the measured temperature values in a system to determine if the temperature needs to be raised or lowered to reach 20 degrees Celsius. A variety of control schemes, including proportional control schemes, proportional integral, proportional integral derivative, fuzzy logic control schemes are known for controlling a process parameter based on a setpoint.
Many existing controllers only have one or a limited number of analog ports available over which to send or receive a setpoint signal. For a controller that asserts analog setpoints to other controllers, this limits the number of devices it can control. In other words, the number of slave controllers to which a particular master controller can assert setpoints is limited to the number of analog ports at the master controller. Additionally, for each controller to which a setpoint is asserted, a separate analog communications link is required.